A matrix radiation detector comprises a matrix of pixels and an electronic circuit forming reading means. Each pixel comprises a photosensitive element generating electrical charges in proportion to the received quantity of photons. These electrical charges, also called photocharges, are processed by the reading means in order to supply an information item representative of the quantity of photons received by each photosensitive element. The use of the CMOS technology has made it possible to integrate the reading means in each pixel. Thus, the electrical charges can be converted into digital signals within the pixels themselves to simplify the transfer of the detection result to outside the matrix. One current solution for producing the reading means is to use a circuit that operates by integration of the electrical charges. This integration circuit comprises an integration capacitance receiving the charges from the photosensitive element, a threshold comparator, a counter and a counter-charge injection circuit. During an exposure phase, the arrival of electrical charges on the integration capacitance brings about a drop in the voltage at its terminals. During an electrical charge reading phase, the threshold comparator switches over a certain number of times, as long as the voltage at the terminals of the integration capacitance is below a threshold voltage. Each switching over of the comparator increments the counter by one unit and commands the injection circuit to inject a packet of counter-charges, the quantity Q0 of which is gauged. The counter is thus incremented by the number of packets of charges necessary to bring a voltage above the threshold voltage at the terminals of the integration capacitance. Generally the injection of counter-charges is performed as the photocharges are collected, a counter determining the number of switchovers of the comparator, in order to estimate the total quantity of charges injected. The reading then corresponds to the reading of the content of the counters. The number of incrementations of the counter provides a numeric value representative of the quantity of photons received by the photosensitive element.
The counter-charge injection circuit is a critical element of the integration circuit. In effect, the accuracy of the measurement relies on the gauging of the quantity Q0 of counter-charges. On the one hand, the quantity Q0 of counter-charges has to be relatively small since it corresponds to the charge quantification pitch; on the other hand, this quantity has to be identical for each packet of counter-charges since it quantifies the charges received by the integration capacitance.
Now, in the current counter-charge injection circuits, the quantity of charges injected on each switchover of the comparator can fluctuate. In effect, these circuits comprise field-effect transistors, the channels of which are effected by a random noise called RTS which stands for “random telegraph signal”. The random nature of this noise influences the quantity of counter-charges injected: the injections are affected by this noise, all differently to one another. So, when trying to estimate the total charge injected by a certain number of injections, exactly how the injections have been affected is unknown.
For example, a counter-charge injection circuit often comprises two field-effect transistors (FET) connected in series and a capacitor connected between the link point of the transistors and a fixed voltage, for example the ground. A first transistor makes it possible to charge the capacitor to a first voltage value, called charge voltage, controlled by the gate voltage of this transistor. The second transistor makes it possible to discharge the capacitor to a second voltage value, called discharge voltage, controlled by the gate voltage of this transistor. The quantity Q0 of counter-charges injected from the capacitor to the integration capacitor of the integration circuit is a function of the value of the capacitance of the capacitor and of the difference between the charge and discharge voltages. However, the charge and discharge voltages cannot be directly deduced from the gate voltages of the transistors. The charge and discharge voltages correspond to the internal potentials of the transistors, which are not known accurately because of the RTS noise due to the trapping of the charges in the channel of each transistor. This RTS noise is all the greater when the components have dimensions reduced in order to generate relatively low quantities Q0 of counter-charges. In practice, this RTS noise modifies the value of the quantity Q0 by a few percentage points. This modification is reflected directly on the assessment of the quantity of photons received, and therefore on the quality of the image obtained. Now, such an error is generally detrimental, particularly in the field of medical imaging.
Solutions for remedying the abovementioned drawbacks do exist. They consist in accurately determining the quantity of counter-charges injected to assess the quantity of charges generated by a photosensitive element. Thus the quantity of charges injected on each switchover of the comparator is controlled. The measurement of the total quantity of the charge collected by a detector is then improved, which increases the accuracy of the measurement.
This technique works well when the quantity Q0 of counter-charges is significant. However, when trying to access smaller charges Q0 (typically of the order of 100 elementary charges injected, that is to say electrons or holes, or less), drawbacks appear. For example, the injected charge Q0 can depend on a potential difference that is variable because of the technological dispersions. The value of this potential difference has to be greater than its variation for the dispersions of Q0 to be acceptable. In other words, the variation of the potential difference has to be negligible compared to the potential difference itself. Furthermore, to obtain small Q0 values, small transistors are used which are more sensitive to the RTS noise. This noise generates variations of threshold voltages, of variable durations. Correcting these variations requires complex devices.
This problem is mentioned in the patent application FR2977413.